Resistance is linear--it's the effects that are tiered.
First: If you read nothing else on this page, read that little bit under the title.
Why Is Spell Resistance Confusing?
Blame Blizzard, I guess.
- See this page? See the graph at the bottom?
- Now, log into your game, bring up your character info, and look at those resistance tooltips on your paper doll. See how it says 'none'/'poor'/'fair'/'good'/'very good'?
That graph and that tooltip have led to the formation of a large segment of the player population that believes that resistances are tiered--that is, you only need to hit certain resistance score thresholds in order to have a sudden 'jump' in your damage resisted. If you can only get to 224, for example, you might as well wear 150, because you couldn't hit the next tier!
This belief has no basis in truth, but it's perceived as true because of the effects that a higher resistance score has on you. I'll discuss here exactly how resistance works in WoW, and even give you some projections and test data to corroborate it.
References
Just so you know I'm not pulling this out of the nether, the information presented here comes from the Blizzard site and the following sources:
Magical Resistance formulas from WoWWiki
Tseric confirming that resistance is linear
In case the Blizzard forum link disappears:
I've always been told and the website has a chart of the values being in steps of 50% at a time etc...
http://www.worldofwarcraft.com/info/basics/resistances.html
The Question: Is it just as good to be at Fire Resist for example at 100 as it is to be at 135... As the next breaking point is 150.
So if youre going to put on FR gear if you don't break 150 you may as well scale back to 100 and so on. 230 you may as well be at 200 etc.
I would suspect not and being at 230 should give me a bit better restiance then 200 naturally but i've heard this debate to many times to not look into it. (by Vince @ Windrunner)Tseric's response:
The extra 35 points will make a difference. The resist tables are not so blocky as to be only significant in increments of 50.
Ten extra resistance may shave only a few points of damage off of a same-level target, but every bit counts ;)
If you read all this and the Blizzard page carefully you will 'get it'. If not, no problem. Maths below!
How Resistance Is Calculated
There are two types of checks to see whether you resist something. One is for 'white resist' (the spell missed or is deflected) and one is for 'yellow resist' (you resisted part or all of the damage; you actually only see the 'yellow resist' if you resist it all).
There are also two types of spells: binary spells and unary spells (which sounds dumb, so call them direct-damage spells).
Binary Spells
Binary spells have an extra effects attached to them--for example, a Frostbolt also snares. When you're hit by a binary spell, you either take no damage (it missed, a 'white resist') or take full damage--no in-between.
To determine whether you get hit or not, a spell hit chance is calculated. If you're the same level, the caster's base chance to hit you is 96%. If you're three levels below a monster (like a raid boss), that chance to hit drops to 83%. The scale's a bit more forgiving in PVP.
That chance to hit is then multiplied by (1-target's resistance%) to get the final hit chance. (1-resist%) is the same as saying 'the chance you will not resist'.
Example 1: Say you're L60 and have 100 frost resist. That's 25% resistance against level 60 attacks. If some L60 mage shoots a frostbolt at you with no +hit% gear on, his chance to hit you is (.96 * (1-.25)) = .72, or 72%. He misses you more than a quarter of the time! When he *does* hit you, though, you always take all the damage (and the snare).
Example 2: Ragnaros the Firelord yells 'TASTE THE FLAMES OF SULFURON!' as he tries to hit Tankard (L60, 315 FR) with the Wrath of Ragnaros, a binary fire spell. With a 3-level advantage, Rag's chance to hit is 99% (and caps at that), but because Tankard has 75% FR, this raid boss only has a (.99 * (1-.75)) = 24.75% chance to hit him successfully with it.
Summary: For a binary spell, your level difference and your resistance score affect the chance that the spell will hit you for full damage (and the debuff). Level and resistance do not affect how much damage a binary spell hits you for if it lands.
Direct-Damage Spells
Direct damage (DD) spells are your standard bolts and other spells that only do damage. You can dodge these (white resist), or if you get hit by them you can resist none (0%), some (25%/50%/75%), or all ('yellow resist') of the damage.
For a DD spell, whether you are hit or not is level-based, like before. The difference is that your resistance is not taken into account yet.
Example 1: This time, you're L60 and the L60 mage fires a fireball at you. Fortunately, you put on 100 FR. His chance to hit you with this spell is just the base chance--96%.Uh-oh! He'll only have a white resist 4% of the time.
Once you are hit, your resistance score against the caster's level affects the chances that you will resist some or all of the damage. Your level does not have anything to do with this check--only your resistance score versus his level.
The maximum resistance that you ever need against someone is (caster's level*5). This works at any level. Ragnaros (L63) needs 315FR; 300FR is good enough in L60 PVP; 150 is good enough for L30; 5 will be enough for squirrel mages.
Resistance caps at 75% average mitigation, when your resistance score is (caster level * 5).
Since most raid bosses are level 63, you need 315 of the appropriate resistance if they use magic.
Effects of Resistance: Average Damage Resisted
The average % of damage resisted (I'm going to call it ADR% from now on) is a fixed ratio; as your resistance score goes up, the ADR% also goes up smoothly from 0% to 75%. Against level 60 attacks, 1 point of resist is worth .25% average damage resisted.
The average percent of damage resisted looks like this:
ADR% = (Your resistance/(caster's level * 5)) * .75
'Well now hold on', you might say; 'Are you saying that, oh, if I have 252 FR I will resist 63% of the damage? That's not true, I only resist 50% or 75% most of the time. So resistance has to be tiered!'
Yes, as anyone with a combatlog (or eyes) knows, you don't always resist the same amount of damage. In fact, you never do. You resist none, 25%, 50%, 75%, or all of the damage. This is the game's way of making resisted damage more interesting than 'blah I always resist 32.333% repeating of course'. In other words:
The effect of resisting a spell is tiered. You take damage in 25% tiers.
This is not the same as resistance itself being tiered. Resistance itself is linear.
The ADR% calculation is actually an average of your chances to reach the separate results of (0/25/50/75/100)% resists. That means that if you add all the separate chances together at a certain resist score (like 120 resist vs. L60), the amount of damage they'd resist equals whatever the ADR% is (30% in this case).
Over time, given the same enemy and the same resistance score, the average amount of damage you resist approaches the ADR% value for that resistance.
If you don't get it, suffice it to say that:
At low resists you're more likely to get 0%/25%/... resists and at high resists you're more likely to get .../75%/full resists.
Example 2: Let's say a L60 mage with +4% spell hit casts fireballs at you from now until the end of time. You have 252 FR (that's 63% ADR% against level 60). He's always going to hit you (no white resist); of those hits, only about 1% (rounded up) should hit you for full damage. 12% of the time you'll resist 25%, 35.5% of the time you'll resist half, 37.5% of the time you'll resist 75%, and 14% of the time you'll resist it all. Add those chances up and you get 1+12+35.5+37.5+14 = 100%--all possibilities are covered.If you added up all the actual damage resisted, it would come out to 63% (note: This example will not because I rounded). But it makes sense, in any case; as your resistance increases, you're more likely to resist higher amounts.
Summary: For a direct-damage spell, your level difference affects the base chance that the spell will hit you. Your resistance versus the caster's level * 5 determines the chances that the spell will hit you for no, 25%, 50%, 75%, or 100% damage. Every point of resistance up to the caster level*5 cap improves your chance that you will resist more of an incoming damaging spell.
I Don't Believe You. Show Me Proof!
Healthy skepticism is good. If you don't want to believe Blizzard's pages, or the CMs, or me, I guess proof is the only way to convince you. On the next page, I'll answer some common misconceptions and show some trial data.